One of the fundamental activities of any software system design is defining the interfaces between the components of the system. Because Java's interface construct allows you to define an abstract interface without specifying any implementation, a major activity of any Java program design is "figuring out what the interfaces are." This article looks at the motivation behind the Java interface and gives guidelines on how to make the most of this important part of Java.

Deciphering the interface

Almost two years ago, I wrote a chapter on the Java interface and asked a few friends who know C++ to review it. In this chapter, which is now part of my Java course reader Inner Java (see Resources), I presented interfaces primarily as a special kind of multiple inheritance: multiple inheritance of interface (the object-oriented concept) without multiple inheritance of implementation. One reviewer told me that, although she understood the mechanics of the Java interface after reading my chapter, she didn't really "get the point" of them. Exactly how, she asked me, were Java's interfaces an improvement over C++'s multiple inheritance mechanism? At the time I wasn't able to answer her question to her satisfaction, primarily because in those days I hadn't quite gotten the point of interfaces myself.

Although I had to work with Java for quite a while before I felt I was able to explain the significance of the interface, I noticed one difference right away between Java's interface and C++'s multiple inheritance. Prior to the advent of Java, I spent five years programming in C++, and in all that time I had never once used multiple inheritance. Multiple inheritance wasn't against my religion exactly, I just never encountered a C++ design situation where I felt it made sense. When I started working with Java, what first jumped out at me about interfaces was how often they were useful to me. In contrast to multiple inheritance in C++, which in five years I never used, I was using Java's interfaces all the time.

So given how often I found interfaces useful when I began working with Java, I knew something was going on. But what, exactly? Could Java's interface be solving an inherent problem in traditional multiple inheritance? Was multiple inheritance of interface somehow intrinsically better than plain, old multiple inheritance?

Interfaces and the 'diamond problem'

One justification of interfaces that I had heard early on was that they solved the "diamond problem" of traditional multiple inheritance. The diamond problem is an ambiguity that can occur when a class multiply inherits from two classes that both descend from a common superclass. For example, in Michael Crichton's novel Jurassic Park, scientists combine dinosaur DNA with DNA from modern frogs to get an animal that resembled a dinosaur but in some ways acted like a frog. At the end of the novel, the heros of the story stumble on dinosaur eggs. The dinosaurs, which were all created female to prevent fraternization in the wild, were reproducing. Chrichton attributed this miracle of love to the snippets of frog DNA the scientists had used to fill in missing pieces of the dinosaur DNA. In frog populations dominated by one sex, Chrichton says, some frogs of the dominant sex may spontaneously change their sex. (Although this seems like a good thing for the survival of the frog species, it must be terribly confusing for the individual frogs involved.) The dinosaurs in Jurassic Park had inadvertently inherited this spontaneous sex-change behavior from their frog ancestry, with tragic consequences.

This Jurassic Park scenario potentially could be represented by the following inheritance hierarchy:

Figure 1. Multiple inheritance in Jurassic Park

The diamond problem can arise in inheritance hierarchies like the one shown in Figure 1. In fact, the diamond problem gets its name from the diamond shape of such an inheritance hierarchy. One way the diamond problem can arise in the Jurassic Park hierarchy is if both Dinosaur and Frog, but not Frogosaur, override a method declared in Animal. Here's what the code might look like if Java supported traditional multiple inheritance:

The diamond problem rears its ugly head when someone tries to invoke talk() on a Frogosaur object from an Animal reference, as in:

Animal animal = new Frogosaur();
animal.talk();

Because of the ambiguity caused by the diamond problem, it isn't clear whether the runtime system should invoke Frog's or Dinosaur's implementation of talk(). Will a Frogosaur croak "Ribbit, Ribbit." or sing "Oh, I'm a dinosaur and I'm okay..."?

The diamond problem would also arise if Animal had declared a public instance variable, which Frogosaur would then have inherited from both Dinosaur and Frog. When referring to this variable in a Frogosaur object, which copy of the variable -- Frog's or Dinosaur's -- would be selected? Or, perhaps, would there be only one copy of the variable in a Frogosaur object?

In Java, interfaces solve all these ambiguities caused by the diamond problem. Through interfaces, Java allows multiple inheritance of interface but not of implementation. Implementation, which includes instance variables and method implementations, is always singly inherited. As a result, confusion will never arise in Java over which inherited instance variable or method implementation to use.

Interfaces and polymorphism

In my quest to understand the interface, the diamond problem explanation made some sense to me, but it didn't really satisfy me. Sure, the interface represented Java's way of dealing with the diamond problem, but was that the key insight into the interface? And how did this explanation help me understand how to use interfaces in my programs and designs?

As time went by I began to believe that the key insight into the interface was not so much about multiple inheritance as it was about polymorphism (see the explanation of this term below). The interface lets you take greater advantage of polymorphism in your designs, which in turn helps you make your software more flexible.

Ultimately, I decided that the "point" of the interface was:

Java's interface gives you more polymorphism than you can get with singly inherited families of classes, without the "burden" of multiple inheritance of implementation.

A refresher on polymorphism

This section will present a quick refresher on the meaning of polymorphism. If you are already comfortable with this fancy word, feel free to skip to the next section, "Getting more polymorphism."

Polymorphism means using a superclass variable to refer to a subclass object. For example, consider this simple inheritance hierarchy and code:

abstract class Animal {

abstract void talk();
}

class Dog extends Animal {

void talk() {
System.out.println("Woof!");
}
}

class Cat extends Animal {

void talk() {
System.out.println("Meow.");
}
}

Given this inheritance hierarchy, polymorphism allows you to hold a reference to a Dog object in a variable of type Animal, as in:

Animal animal = new Dog();

The word polymorphism is based on Greek roots that mean "many shapes." Here, a class has many forms: that of the class and any of its subclasses. An Animal, for example, can look like a Dog or a Cat or any other subclass of Animal.

Polymorphism in Java is made possible by dynamic binding, the mechanism by which the Java virtual machine (JVM) selects a method implementation to invoke based on the method descriptor (the method's name and the number and types of its arguments) and the class of the object upon which the method was invoked. For example, the makeItTalk() method shown below accepts an Animal reference as a parameter and invokes talk() on that reference:

class Interrogator {

static void makeItTalk(Animal subject) {
subject.talk();
}
}

At compile time, the compiler doesn't know exactly which class of object will be passed to makeItTalk() at runtime. It only knows that the object will be some subclass of Animal. Furthermore, the compiler doesn't know exactly which implementation of talk() should be invoked at runtime.

As mentioned above, dynamic binding means the JVM will decide at runtime which method to invoke based on the class of the object. If the object is a Dog, the JVM will invoke Dog's implementation of the method, which says, "Woof!". If the object is a Cat, the JVM will invoke Cat's implementation of the method, which says, "Meow!". Dynamic binding is the mechanism that makes polymorphism, the "subsitutability" of a subclass for a superclass, possible.

Polymorphism helps make programs more flexible, because at some future time, you can add another subclass to the Animal family, and the makeItTalk() method will still work. If, for example, you later add a Bird class:

class Bird extends Animal {

void talk() {

System.out.println("Tweet, tweet!");
}
}

you can pass a Bird object to the unchanged makeItTalk() method, and it will say, "Tweet, tweet!".

Interfaces give you more polymorphism than singly inherited families of classes, because with interfaces you don't have to make everything fit into one family of classes. For example:

interface Talkative {

void talk();
}

abstract class Animal implements Talkative {

abstract public void talk();
}

class Dog extends Animal {

public void talk() {
System.out.println("Woof!");
}
}

class Cat extends Animal {

public void talk() {
System.out.println("Meow.");
}
}

class Interrogator {

static void makeItTalk(Talkative subject) {
subject.talk();
}
}

Given this set of classes and interfaces, later you can add a new class to a completely different family of classes and still pass instances of the new class to makeItTalk(). For example, imagine you add a new CuckooClock class to an already existing Clock family:

class Clock {
}

class CuckooClock implements Talkative {

public void talk() {
System.out.println("Cuckoo, cuckoo!");
}
}

Because CuckooClock implements the Talkative interface, you can pass a CuckooClock object to the makeItTalk() method:

With single inheritance only, you'd either have to somehow fit CuckooClock into the Animal family, or not use polymorphism. With interfaces, any class in any family can implement Talkative and be passed to makeItTalk(). This is why I say interfaces give you more polymorphism than you can get with singly inherited families of classes.

The 'burden' of implementation inheritance

Okay, my "more polymorphism" claim above is fairly straightforward and was probably obvious to many readers, but what do I mean by, "without the burden of multiple inheritance of implementation?" In particular, exactly how is multiple inheritance of implementation a burden?

As I see it, the burden of multiple inheritance of implementation is basically inflexibility. And this inflexibility maps directly to the inflexibility of inheritance as compared to composition.

By composition, I simply mean using instance variables that are references to other objects. For example, in the following code, class Apple is related to class Fruit by composition, because Apple has an instance variable that holds a reference to a Fruit object:

class Fruit {

//...
}

class Apple {

private Fruit fruit = new Fruit();
//...
}

In this example, Apple is what I call the front-end class and Fruit is what I call the back-end class. In a composition relationship, the front-end class holds a reference in one of its instance variables to a back-end class.

In last month's edition of my Design Techniques column, I compared composition with inheritance. My conclusion was that composition -- at a potential cost in some performance efficiency -- usually yielded more flexible code. I identified the following flexibility advantages for composition:

It's easier to change classes involved in a composition relationship than it is to change classes involved in an inheritance relationship.

Composition allows you to delay the creation of back-end objects until (and unless) they're needed. It also allows you to change the back-end objects dynamically throughout the lifetime of the front-end object. With inheritance, you get the image of the superclass in your subclass object image as soon as the subclass is created, and it remains part of the subclass object throughout the lifetime of the subclass.

The one flexibility advantage I identified for inheritance was:

It's easier to add new subclasses (inheritance) than it is to add new front-end classes (composition), because inheritance comes with polymorphism. If you have a bit of code that relies only on a superclass interface, that code can work with a new subclass without change. This isn't true of composition, unless you use composition with interfaces.